The present invention relates to the field of microwave energy absorbing materials, and more particularly to such materials which employ orientationally mobile, polar icosahedral molecular units as the source of dielectric loss at microwave frequencies.
A variety of host materials loaded with either carbon or ferrite-type particles comprise the great majority of microwave-absorbing materials currently in use. The carbon-loaded (or carbon) materials absorb microwave radiation through dielectric loss, while the ferrite-type-loaded (or ferrite materials absorb through magnetic loss. Although there have been significant improvements in the host matrices for the carbon and ferrite-type particles, there is a need for additional microwave-absorbing particles/clusters/molecules that can be incorporated into such host matrices.
This need is best stated by examining the deficiencies of the carbon and ferrite materials in regard to weight, mechanical strength, impedance matching, chemical tuning, and IR and visible transparency.
Because of their higher microwave absorptivities and better impedance-matching properties, the ferrite materials are generally preferred over the carbon materials. However, the ferrite materials are much heavier than the carbon materials. This difference remains an issue even though the ferrite materials can be applied as thinner layers than the carbon materials. Thus, other lightweight, microwave-absorbing materials are needed.
Both the ferrite and carbon materials are blends, and therefore possess poor mechanical strength. Microwave-absorbing polymers would be of great interest because their mechanical stability is generally much greater than that of blends.
A material with a high dielectric or a magnetic loss at microwave frequencies does not automatically make it a leading candidate as a microwave absorber. The reason is that most of the microwave radiation is reflected at the air-material interface, rather than absorbed, for high loss materials like electron-conductors. A large number of ingenious air-material impedance-matching strategies have been devised for carbon materials, e.g., loaded foams or multilayer stacks with a different amount of carbon loading in each layer. The ferrite materials are rather special in that they come close to the case where .mu.=.epsilon., which results in very low reflection at normal incidence. Additional air-material, impedance-matching strategies are always of interest for new microwave-absorbing materials, as well as for the established carbon and ferrite materials.
Chemical tuning refers to the possibility of modifying the microwave dielectric and/or magnetic properties of a microwave-absorbing material by changing the chemical structure of the material by varying the concentration of electron-donating or electron-withdrawing moieties, or other chemically-related alterations of the material. Both the carbon and ferrite materials are limited in regard to the chemical tuning of their dielectric and magnetic properties. Additional microwave-absorbing materials are needed that can be chemically tuned in regard to their dielectric and/or magnetic loss, and impedance matching.
Both the carbon and ferrite materials are opaque in the IR and visible spectral regions. Microwave-absorbing materials are needed that are transparent in these regions. Of course, this requirement is most relevant in the area of window applications.
Noble metal thin films and thin films made from doped wide band-gap semiconductors are widely used at present as microwave-reflecting coatings that are transparent in the visible spectral region. These coatings are well-characterized and can be applied to a variety of surfaces including silica glass and plastics. One drawback with these materials is that their electron conductivities make them opaque in the IR spectral region. If these materials are made non-conducting, as is the case for the zinc sulfide wide band-gap semiconductor, then transparency is obtained in both the IR and visible spectral regions, but no attenuation in the microwave spectral region. Narrow band-gap semiconductors, e.g., silicon, have the potential to serve as IR-transparent substrates into which microwave-attenuating circuit elements can be etched. This approach appears to be rather expensive at the present time.
Electron-conducting polymers have spectroscopic properties similar to the doped wide band-gap semiconductors, but have the advantage of polymer processibility. The low visible and IR transparencies of this polymer remain an issue.
It is therefore an object of the invention to provide a microwave energy absorbing material which has high dielectric loss, high mechanical strength, and is light-weight.
A further object is to provide a microwave energy absorbing material which is impedance-matchable to air.
Other objects include providing a microwave-energy absorbing material which has chemically-tunable dielectric properties, and/or which is IR-transparent, and/or which is transparent to visible light.
Such a material would be useful in the reduction of side-lobe reflections from surfaces in general, and in IR and visible windows requiring microwave absorption.